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The present invention relates to a fiber grating system, particularly to a dual fiber Bragg grating strain sensor system for operating in a temperature-independent mode.
Prior Arts
Intracore fiber Bragg gratings (FBGs) have become attractive optical components for sensing because of their compactness and wavelength-encoding capability. When the FBGs are applied as sensors, the measurands can be temperature, strain, pressure, magnetic field, and others[1,2]. These measurands are determined by detecting the wavelength shift (i.e., the Bragg wavelength shift) of the light backreflected from the FBGs. In either a single-point sensor or a multiplexed sensor system, the Bragg wavelength shift can be measured by employing a wavelength interrogation scheme with a tunable filter[3], an interferometer[4], a diffraction grating and CCD spectrometer[5], a frequency-locking circuit[6], or a frequency-modulated multimode laser[7].
Strain measurement with FBGs has been an important subject in applications associated with monitoring stressed composite materials. A sensor system with an array of wavelength-multiplexed FBGs has been widely proposed for strain measurement in a smart structure[8,9]. Many FBG strain sensor systems have been further developed to operate in a temperature-independent mode. For an ac strain measurement, use of a single FBG in conjunction with a local locking circuit can suffice to discriminate the ac strain response from the slowly varying temperature response. However, it is not possible to do so with only one FBG used for measuring a static or quasi-static strain. Various methods have been proposed to resolve this situation. A frequently used method is to utilize a pair of FBGs written into the same location to form a superstructure that exhibits different sensitivities to temperature and strain[10,11]. Dual FBGs could also be structured to form a cavity for simultaneous measurement of strain and temperature[12]. Dual fiber Bragg gratings can also combine with a long period fiber grating to discriminate temperature and strain[13]. These methods require calibration for the two sensitivities of each FBG in advance, resulting in some complexity in application. However, note that some temperature independent strain measurements have been carried out without such calibration by simply nullifying the strain sensitivity of one FBG while keeping the same temperature sensitivity for both FBGs. A recent case of temperature-independent strain sensing exemplified this by employing a chirped FBG partially embedded in a glass tube to obtain a direct optical power-detection scheme[14]. Although simple in structure, this detection scheme has not yet provided a solution to linear and accurate strain measurement.
Objects of this Invention
The present invention provides a temperature-independent fiber Bragg grating strain sensor system with specific sensor(s) to improve the strain-sensing interfered by temperature.
In the aspect of detection, this invention further incorporates the use of optical power detector in the detection port to measure the variation of strain; thus, it is not necessary to utilize an expensive equipment for measuring fine variation of Bragg wavelength and neither to utilize complex digital signal process chips for signals from different locations. Instead, this invention adapts cheap power reading unit(s) to detect the optical power responded from the sensor unit(s) at different locations. Comparing with the prior arts, the present invention costs much lower. Besides, this invention can be a temperature-independent multipoint strain sensor system, which can be obtained in accordance with the result of experiment. In another word, the sensor unit(s) can sense the variation of strain in multiple locations without cross talk. Furthermore, the system provided in this invention also utilizes an optical power splitter to split the broadband light for a plurality of power reading units; thus, it can sense multipoint strain variation in a temperature independent mode.
The present invention provides a relatively cheap and easy way to produce temperature-independent multipoint strain sensor system without separating the reflection spectra of fiber Bragg gratings in different sensor units.
The main object of the present invention is to provide a temperature-independent strain sensor system.
The other object of this invention is to provide a relatively cheap strain sensor system.
Another object of this invention is to provide a temperature-independent multipoint strain sensor system.
The present invention provides a dual fiber Bragg grating strain sensor system, which comprises a broadband light source, an optical power reading unit, and a sensor unit having a pair of fiber Bragg gratings (FBG""s). It is required that the reflection spectra of these FBGs be either identical or slightly overlapped, and that the pair of FBGs be structured so that only one is insusceptible to strain effect while both have the same temperature sensitivity. The latter requirement can be realized by embedding one of the FBGs in a thin glass tube sealed with glue at both ends and keeping the other FBG free but placing it adjacent to the tubed FBG. This design is particularly important when the sensor is embedded in a stressed material to insure that both FBGs have an identical thermal effect but experience different strain effects. The present invention also provides an optical power detection scheme for temperature-independent strain measurements.
The following detailed description and figures are provided to help understand the objects and features of the present invention.